Continuously variable transmissions of the class which may be broadly characterized as that wherein a belt couples a pair of pulleys, each of which can assume a more or less continuous range of effective diameters, generally fall into two categories; viz.: (a) those systems employing V-belts or variations thereof (such as link belts or chains) for transmitting power from one pulley to the other and (b) those systems employing flat, flexible belts between the variable diameter pulleys.
Those skilled in the art have come to appreciate that CVT's employing flat, flexible belts enjoy significant fundamental advantages over those systems employing V-belts. In the case of the latter, the belts are composed of various compositions and have a trapezoidal cross section, the belt transmitting rotary motion at one speed from a source of power (such as an engine or motor) to an output shaft at another speed, the speed ratio being varied in a continuous fashion from a minimum to a maximum as dependent on the geometry of the belt and the pulley system. The V-belt is compressed between smooth, conical sheave sections in each of the two pulleys by external axial forces acting on the sections to apply compression to the belt giving friction between the sides of the belt in the sheave sections to prevent slippage. In operation, a force unbalance caused by changes in the axial loading of the sheave sections causes the V-belt to change its radial positions in the two pulleys until a force balance is achieved or a limit range stop is reached.
For a large transmitted torque, the required axial forces exerted on the sheaves result in large compressive forces on the V-belt which requires that the belt have a substantial thickness to prevent its axial collapse or failure. This increase in thickness increases the belt's centrifugal force and causes higher belt tension load. In addition, as the belt thickness increases, the pulley size must be increased due to higher stress loads at a given design minimum pulley radius. Further, the typical V-belt must continuously "pull out" from the compressive sheave load on leaving each pulley which results in significant friction losses and belt fatigue which adversely affects the overall efficiency of the system and the operating life of the belt. Consequently, although variable speed pulley drives have successfully used V-belts in a wide range of applications, they have been severely limited in their power capabilities for more competitive smaller size equipment.
As a result of these inherent drawbacks to the use of V-belts in continuously variable transmissions, a second category has developed which may broadly be designated as flat belt drive continuously variable transmissions. As the name suggests, flat belts are employed between driven and driving pulley assemblies which are dynamically individually variable in diameter to obtain the sought-after ratio changes. No axial movement between the two pulley rims is necessary. On the other hand, it is necessary to somehow effect the diametric variations of the individual pulley assemblies, and in one particularly effective system, this function is achieved by causing a circular array of drive elements in each pulley to translate radially inwardly or outwardly in concert as may be appropriate to obtain a given effective diameter of the pulley assembly. Variable speed flat belt transmissions of this particular type, and their associated control systems, are disclosed in U.S. Pat. Nos. 4,024,772; 4,295,836; 4,591,351 and 4,714,452 as well as United States patent applications Ser. No. 051,922, filed May 19, 1987, and now U.S. Pat. No. 4,765,996; and Ser. No. 132,783, filed Dec. 14, 1987, all issued to Emerson L. Kumm. In all but the first patent enumerated above, the subject variable diameter pulley components have included a pair of pulley sheaves between which there extends a series of belt engaging elements that are simultaneously moved both radially and circumferentially. In one exemplary construction, there is a series of twenty-four belt engaging elements such that an angle of fifteen degrees extends between runs of the belt coming off tangentially from one belt engaging element compared to that of an immediately adjacent belt engaging element.
Each pulley assembly includes two sets of two disks (designated, respectively, the inner guideway disk and the outer guideway disk in each pair) which are parallel to each other with the inner and outer guideway disks of each set being disposed immediately adjacent one another. Each of the guideway disks of an adjacent pair has a series of spiral grooves or guideways with the guideways of the pair oriented in the opposite sense such that the ends of the belt engaging elements are captured at the intersections of the spiral guideways. Thus, radial adjustment of the belt engaging elements may be achieved by bringing about transient relative rotation between the inner and outer guideway disks to change their angular relationship, this operation being, of course, carried out simultaneously and in coordination at both sets of guideway disks of a pulley assembly.
The disclosure of the above-identified patents and patent applications to Emerson L. Kumm include control systems for effectively establishing the mutual angular relationships between the inner and outer guideway disks of each pulley assembly in a CVT employed, for example, in a straightforward vehicular automatic transmission application. Those skilled in the art will appreciate that extremely precise adjustment of the speed ratio between the pulley assemblies is not required in such an environment and is not achieved by the prior art control systems. Similarly, manual adjustment to a precise speed ratio is neither required nor obtained.
However, there are other vehicular and non-vehicular applications for CVT's in which it is very desirable to obtain a very precise adjustment to the speed ratio. For example, in the so-called geared neutral configuration for a vehicle automatic transmission, it is necessary that, under certain conditions, the speed ratio be established and maintained at a precise value or regenerative torque buildup in the transmission can cause the failure of gears, etc. As another example, there are non-vehicular CVT applications, such as in driving precision machine tools, in which it is advantageous to have the facility for manually "cranking in" and thereafter closely maintaining a narrowly defined speed ratio. The prior art control systems for flat belt CVT's, however, have simply not been able to achieve and/or maintain the speed ratio to the precision required for these applications such that the use of a flat belt CVT in such applications has been heretofore precluded. My invention is directed to achieving very close control of the speed ratio in a flat belt CVT.